The unlicensed frequency bands of the electromagnetic spectrum are shared with a variety of systems. For example wireless local area networks (WLANs) and Bluetooth networks utilize the Industrial, Scientific, and Medical (ISM) band between 2400 MHz and 2483.5 MHz. In addition, microwave ovens, and harmonics of cellular telephony transmissions (such as GSM 850 and IS-95 transmissions) may cause interference in such unlicensed bands.
It is useful for short-range communications systems (e.g., Bluetooth and IEEE 802.11 networks) to recognize the traffic of neighboring systems. When constant regular transmissions by the interferers in the same channel are recognized, a network or device may avoid collisions with these regular transmissions by scheduling its own transmissions to be within other unoccupied channels or to be at times when other systems do not occupy the channel. Such avoidance reduces the number of retransmissions due to collisions, thereby enabling more efficient use of the band.
Bluetooth defines a short-range radio network, originally intended as a cable replacement. It can be used to create ad hoc networks of up to eight devices, where one device is referred to as a master device. The other devices are referred to as slave devices. The slave devices can communicate with the master device and with each other via the master device. Bluetooth devices are designed to find other Bluetooth devices within their communications range and to discover what services they offer.
Bluetooth networks may utilize 79 channels. Each of these channels has a 1 MHz bandwidth. To enhance robustness, Bluetooth networks perform frequency hopping among all or some of these 79 channels.
WLANs are local area networks that employ high-frequency radio waves rather than wires to exchange information between devices. IEEE 802.11 refers to a family of WLAN standards developed by the IEEE. In general, WLANs in the IEEE 802.11 family provide for 1 or 2 Mbps transmission in the 2.4 GHz band using either frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS) transmission techniques. Within the IEEE 802.11 family are the IEEE 802.11b and IEEE 802.11g standards, which are collectively referred to herein as IEEE 802.11b/g.
IEEE 802.11b (also referred to as 802.11 High Rate or Wi-Fi) is an extension to IEEE 802.11 and provides for data rates of up to 11 Mbps in the 2.4 GHz band. This provides for wireless functionality that is comparable to Ethernet. IEEE 802.11b employs only DSSS transmission techniques. IEEE 802.11g provides for data rates of up to 54 Mbps in the 2.4 GHz band. For transmitting data at rates above 20 Mbps, IEEE 802.11g employs Orthogonal Frequency Division Multiplexing (OFDM) transmission techniques. However, for transmitting information at rates below 20 Mbps, IEEE 802.11g employs DSSS transmission techniques. The DSSS transmission techniques of IEEE 802.11b/g involve signals that are contained within a 23 MHz wide channel. Several of these 23 MHz channels are within the ISM band.
Current short-range communications systems provide techniques for measuring channel characteristics in a particular frequency band to find interfering systems or devices. However, these techniques are not ideal for collecting statistics of the interferences in the band.
For instance, IEEE 802.11b/g devices employ a carrier sensing technique before transmitting signals. This technique is known as Carrier Sensing Multiple Access/Collision Avoidance (CSMA/CA). CSMA/CA prevents collisions with other transmissions, which have already started. However, such techniques do not prevent collisions when two or more transmissions commence at the same time. Also, other systems that do not employ carrier sensing may commence transmissions while an IEEE 802.11b/g device is transmitting.
To avoid transmitting in channels employed by other systems, Bluetooth employs an adaptive frequency hopping (AFH) technique. With this technique, a frequency hopping Bluetooth device “hops around” channels that are used by other systems. However, before “hopping around” may begin, the devices in the Bluetooth piconet must first identify the static interferences.
Such identification involves measuring electromagnetic energy in the channels available to the Bluetooth piconet. When Bluetooth slave devices perform such measurements, they regularly transmit channel classifications to the master device, which decides which channels may be used for Bluetooth communications. The method to measure and classify the channels is not specified for Bluetooth. Channels can be classified based on received signal strength indication (RSSI) measurements in the slots when the piconet is not transmitting. In slots that the piconet is transmitting, channels are classified based on information regarding received packets, such as bit error or failed packet statistics. Bluetooth channels may also be classified based on a collaborative classification technique. Collaborative classification involves a host knowing other systems employed by the same device and classifies the channels utilized by the other system as “bad.”
A drawback of the above channel measurement techniques for detecting interfering transmissions is that they consume a considerable amount of time, power, and bandwidth. Because the measuring is time consuming, it is difficult to collect interference related information.
In addition, RSSI measurements require additional bandwidth and power consumption. For instance, background RSSI measurements can be made when there are not any transmissions in the network. In Bluetooth, it takes about 25 milliseconds (i.e., 79 times 312.5 microseconds) to measure all of the channels once during each 312.5 microsecond half slot. However, one measurement per channel does not reveal if the interference is static or hopping. Therefore, it takes about 250 milliseconds, if it is assumed that at least 10 measurements are required per channel to detect the static interference.
Because the network may not be able to stop its traffic for 250 milliseconds or even for 25 milliseconds, the actual time to measure the channels can be longer, depending on the utilization of the piconet. In addition, those 10 measurements have to be performed again after a short period to detect if some new static interference source has started transmitting, or if some old interference source has stopped transmitting.
In Bluetooth, the performance of error detection requires at least 100 ms (i.e., 79 times 625 microseconds times 2) to receive a packet in every channel, if the network utilization is 100% and only single-slot packets are used. Accordingly, the time for 10 measurements per channel is at least 1 second. However, if the utilization is not 100%, channel classification takes longer.
When the characteristics of the interfering transmissions are known, more efficient use of the band is possible. Accordingly, techniques are needed for the effective detection of interference sources.